1. Field of the Invention
The invention relates to fuel injection for an internal combustion engine and more specifically to a metering valve for such a system.
2. Description of the Prior Art
The advantages of fuel injection are well known. The degree with which the advantages are obtained is governed greatly by the accuracy and timing flexibility of the metering valve or valves in an injection system and ultimately by the cost of the metering valves and system for controlling the valves. An injection system metering valve for a compression ignition engine should meter the quantity of fuel demanded for engine speed and load, should meter an equal quantity to each cylinder at the optimum time and rate, and should sharply control injection pressure rise and fall to the injection nozzle to avoid nozzle dribble and after injection.
Several different basic types of fuel injection systems have been devised. The most successful of the basic types have been the common rail system and the jerk pump system. Many variations and combinations of the basic systems have also been devised. The basic common rail system employs a single pump for maintaining injection pressure to a common header and one or more metering valves. The rate of fuel metering in such systems is a function of time, since injection pressure is constant. The basic jerk pump system employs one or more jerk pumps which provide both the injection pressure and the metering. The rate of fuel injection in such systems is relatively constant with respect to degrees of engine crankshaft rotation; the rate varies greatly with respect to time; and pressure varies greatly with respect to engine speed.
During the past several years common rail systems have had decreasing success with compression ignition engines operating over a wide speed and load range. Compression ignition engines require high injection pressures and the known types of metering valves capable of accurately metering the high pressure fuel required relatively high, synchronized actuating forces. Engine driven actuating mechanisms provided both. However, they also operate the valving members in the metering valves at speeds proportional to engine speed, i.e., increasing engine speeds caused increasing valving member speeds with respect to time, thereby undesirably reducing the quantity of fuel metered, since metering rate is a function of time in such systems. Varying the size of the metering orifice in the valve as a function of engine speed and load was one method of maintaining and increasing, respectively, the quantity of metered fuel. This was costly and complex, as were other methods. Common rail systems have had a rather high degree of success with spark ignition engines, since such engines require a relatively low pressure for manifold injection, whereby the metering valve may be actuated by a solenoid producing relatively low forces.
Injection systems employing jerk pumps, which combine pumping and metering into a single unit, have had a high degree of success with diesel engines. Such systems may have one combined unit supplying several engine cylinders via a distributor or one unit per engine cylinder. In either case the unit often includes a piston and a bore defining a chamber which is expanded and contracted in response to reciprocating movement of the piston. The piston is reciprocated by an engine drive cam at a speed proportional to engine speed. A variable volume of fuel is trapped in the expanded chamber and then impulsively pushed to an engine cylinder in response to the piston moving in a direction contracting the chamber. Such units have several disadvantages due to the high forces required to raise the trapped fuel volume to the high injection pressure required for a diesel engine: The drive train between the piston and the engine must be designed to withstand high torques. If variable injection timing is required, the drive train must include a sturdy phase change mechanism capable of withstanding the high torques required. The high driving forces cause high side loading of the piston; this loading accelerates wear of the piston and the bore. Injection pressures are lower than ideal at low engine speeds and higher than ideal at high engine speeds, since the piston speed is proportional to engine speed. Leakage of fuel from the trapped volume increases with decreasing engine speed. Rise and fall of the injection pressure is rather slow due to the cyclic pumping of the fuel by the piston.